This invention relates to a curtain coating apparatus which applies various coating composition to a continuously traveling web-shaped support (web) by the curtain coating system, in the production of photographic film, photographic paper, magnetic recording tape, pressure-sensitive recording paper, offset printing plate and the like.
In general, photographic film is manufactured by applying a coating solution comprising a photographic emulsion to a continuously traveling web made of polyethylene terephthalate, followed by drying.
A representative coating apparatus for the coating solution is a curtain coating apparatus. In the curtain coating apparatus, as shown in FIG. 4, one or two or more types of a coating solution 43 are extruded into a film or layered films onto a sliding face 42 of a slide hopper die, and the extruded coating solution 43 film(s) flow down with regulating the width by coating width regulating plates 44 provided on both sides. The coating solution film(s) fall down from the end of the sliding face 42 with keeping both side edges of the film(s) to form a free falling coating solution film (free falling curtain) 46, and the free falling coating solution film is applied to the web 47 as a coating film 48.
In the curtain coating having a sliding face, contraction of the free falling coating solution film is prevented compulsorily by using a pair of edge guides which keep a prescribed coating width. Therefore, various forces act on the stream of the free falling coating solution film around the connection between the coating width regulating plate set of the sliding face and the edge guide, and induce instability.
For example, at the side edges of the free falling coating solution film formed along the edge guide, thinning phenomenon of the film occurs, i.e. thinning and thickening occur caused by the development of a boundary layer, and bending of the free falling coating solution film in the front and rear direction occurs which is called teapot effect. Since the strength of these phenomena greatly depends on the flow rate and viscosity, it is desirable to control these conditions in order to inhibit the occurrence of the tinning phenomenon and the bending phenomenon. However, coating flow rate and viscosity are, in general, restricted according to the type of product, and it is impossible to adjust them freely. As a result, the countermeasure against the occurrence of the thinning phenomenon and the bending phenomenon is taken mechanically, such as the design of a coating width regulating plate and an edge guide.
Under these circumstances, various devices for the coating width regulating plate and the edge guide have been proposed. For example, Japanese Patent KOKAI 51-57734 discloses a film cover composed of two vertical edge guide plates having a width which the edges of a free falling liquid film can follow the passage naturally adjusted by the guide plates Japanese Patent KOKAI 61-245862 discloses a coating apparatus which inhibits disturbance upon film forming by rendering the face width of the vertical face of a stretching means in contact with a side edge of a falling film to the face width corresponding to the thickness of the falling film. Japanese Patent KOKOKU 4-4662 discloses a coating apparatus which improves unevenness of a film thickness on both sides of a free falling curtain by adjusting a guide means of the free falling curtain to a prescribed temperature range. Japanese Patent KOKAI 63-287575 discloses a coating apparatus which is devised so that the stream of a coating solution is not disturbed at the connecting portion between a coating width regulating plate and an edge guide which supports both sides of a film of a coating solution, by processing the section of the edge guide in the same shape as the section of the coating width regulating plate.
However, in the film cover disclosed in Japanese Patent KOKAI 51-57734, the landing point of a curtain film varies by varying the shape of the curtain film according to conditions, such a coating viscosity, flow rate, and the like. Accordingly, when coating was carried out on backing roller, coating surface quality is liable to be influenced by the variation in the vertical distance between a die lip and the landing point. Moreover, it is also a problem that the lower end of a curtain film is liable to be unstable because of not being pinned but being movable forward and backward.
In the coating apparatus disclosed in Japanese Patent KOKAI 61-245862, the shape of the stretching means is special, and must be optimized according to curtain coating conditions. Accordingly, the design of the stretching means is complicated.
In the coating apparatus disclosed in Japanese Patent KOKOKU 4-4662, although the uneven thickness at the side end portions of a free falling curtain film can be improved, the inhibition is small against the generation of streaks caused by the contraction or the teapot phenomenon.
In the coating apparatus disclosed in Japanese Patent KOKAI 63-287575, the inhibition of uneven film thickness at edge guide portions can be improved, but it is desird to further improve the uniformity of the film thickness.
An object of the invention is to provide a curtain coating apparatus which has solved problems of streaks and uneven thickness at side end portions of a free falling film caused by contraction and teapot phenomenon (residence), by changing wet properties at a coating width regulating plate and an edge guide.
The inventors investigated eagerly in order to achieve the above object, and found that, when the contact angle of water with a coating width regulating plate is greater than the contact angle of water with an edge guide, contraction of stream of a coating solution which occurs in the vicinity of the connection between the coating width regulating plate and the edge guide can be minimized and uneven thickness and the generation of streaks caused by the turbulence of stream can be inhibited.
The present invention has been completed by the findings, and provides a curtain coating apparatus which comprises a sliding face on which a coating solution flows down, a coating width regulating plate which is located on each of both sides of the sliding face and regulates the width of the flow of the coating solution, and an edge guide which is connected to the width regulating plate, and keeps both side edges of free falling stream of the coating solution from the sliding face, wherein a contact angle of water with the coating width regulating plate is greater than a contact angle of water with the edge guide.